1. Field of the Invention
Exemplary embodiments of the present invention relate to a technology for transmitting frames to multiple users in a wireless communication system; and, more particularly, to a method for transmitting frames to multiple users while efficiently using an available frequency band as much as possible, in a wireless communication system using a plurality of frequency band segments, and a method for receiving frames.
2. Description of Related Art
In general, a wireless communication system such as a wireless LAN (WLAN) system includes a plurality of access points (AP) which are connected to one another through a network. A plurality of stations (STAs) existing within a basic service set (BSS) managed by one AP simultaneously transmit/receive frames to and from the AP. Meanwhile, an independent BSS (IBSS) includes a plurality of STAs, and one STA transmits/receives a frame at the same time as the other STAs.
A wireless communication system using multiple antennas may acquire an increase in channel capacity according to an increase in the number of antennas.
Accordingly, since the wireless communication system using multiple antennas may increase frequency efficiency, the wireless communication system has been recently adopted in various standards.
Recently, much attention has been paid to not only multiple input multiple output (MIMO) technology for transmitting multiple streams to only one user, but also multi-user MIMO (MU-MIMO) technology for transmitting multiple streams to multiple users.
The MU-MIMO technology, in which an AP eliminates interference between a plurality of STAs and transmits data, may acquire a MU diversity gain as well as an increase in channel capacity of multiple antennas. The MU-MIMO technology may simultaneously transmit multiple streams to multiple users by using the same frequency band. Therefore, the MU-MIMO technology may efficiently increase throughput while increasing the complexity of the AP.
A conventional method for increasing throughput is to increase a frequency band. Such a conventional method uses a large number of expensive and limited frequency bands, but is simpler than the MU-MIMO technology which increases the complexity of an AP without increasing the frequency band. For this reason, a variety of methods which employ the MU-MIMO technology while using variable frequency bands depending on the surrounding conditions have been discussed in recent standards such as the IEEE 802.11ac.
In a wireless communication system which simultaneously transmits multiple antenna streams to a plurality of users while using variable frequency bands, a plurality of frequency band segments may exist. However, when STAs supporting all frequency band segments and STAs supporting only some frequency band segments are mixed in such a wireless communication system which supports the plurality of frequency band segments, an available frequency band may not be utilized as much as possible but be wasted, depending on the frequency band segments of the STAs. For example, a case in which STAs supporting only one 80 MHz band and STAs supporting two 80 MHz bands are mixed may be considered in a wireless communication system which uses an 80 MHz frequency band as one segment. In this case, although an AP secures an available frequency band of 160 MHz, a frequency band of 80 MHz may not be used when the AP communicates with an STA supporting the 80 MHz frequency band, and thus the frequency resource may be wasted.